When to use "in number" and when to use "in numbers"?

When talking about a quantity of people or things, I get 2 phrases "in number" and "in numbers". It's a little bit confusing for me, so I ask for help. These are some examples I got from online dictionaries

• We were eight in number(= there were eight of us).
• Letters of complaint were surprisingly few in number (= there were not many of them).
• The protesters were few in number, but they were very loud.

.

• Nurses are leaving the profession in increasing numbers.
• Newspapers are produced in vast numbers.

Solution 1:

Nurses are leaving the profession in increasing numbers.

means the same thing as

Nurses leaving the profession are increasing in number.

In the first case, "in increasing numbers" is an adverb phrase describing how nurses are leaving the profession, as in "Nurses are leaving the profession without giving notice".

In the second case, "increasing in number" is an adjective phrase describing the nurses that are leaving the profession, as in "Nurses leaving the profession are predominantly experienced."